Method of reducing metal losses in coal slurry transportation pipelines



y 1957 s. BARTHAUER ETAL I 2,791,472

METHOD OF REDUCING METAL LOSSES IN COAL SLURRY TRANSPORTATION PIPELINES Filed May 4, 1954 3 Sheets-Sheet 1 FIG. l

I L AL SLURRY DEVIATERING TERMINAL PUMP PUMP

a: g z- FRANK u. SPELLER g g GERALD L. BARTHAUER m 0: JOHN A. PHINNEY v 8 EDWARD J. wAsP JNVENTORS WATER SLURRY PREPARATION TERMINAL AT ORNEY May 7, 1957 Filed May 4, 1954 G. L. BARTHAUER ETAL 2,791,472 METHOD OF REDUCING METAL LOSSES IN COAL SLURRY TRANSPORTATION PIPELINES 3 Sheets-Sheet 2 \CARBON STEEL A NO mmamou AVERAGE PENETRATION, INCHES PER YEAR CARBON STEEL CHROMATE- CALGON INHIBITION I STAINLESS STEEL \B .No INHIBITION 4 8 l2 l6 2O 24 VELOCITY, FEET PER SECOND FIG. 2

FRANK N. SPELLER GERALD L. BARTHAUER JOHN A. PHINNEY EDWARD J- WASP INVENTORS BY W A w TTORNEY y 7, 1957 G. L. BARTHAUER EIAL 2,791,472 METHOD OF REDUCING METAL LOSSES IN COAL SLURRY TRANSPORTATION PIPELINES Filed May 4, 1954 3 Sheets-Sheet 3 Z LIJ LL] 0: 0 m N I (D DJ 2 m9 mu.

'- Z DJ 0 I! l-l-l m I 2 LL n q '0' N 2 2 0 O 0 W31! use SEOIHONI uouvuuuad WflWlXVW ED WARD J. WASP INVENTORS BY 7: ATTORNEY METHOD OF REDUCING METAL LOSSES IN COAL SLURRY TRANSPORTATION PIPELINES Gerald L. Barthauer and John A. Phinney, Pittsburgh,

Edward J. Wasp, Library, and Frank N. speller, Pittsburgh, Pa., assignors to Pittsburgh Consolidation Coal Company, Pittsburgh, Pa., a corporation of Pennsylvania Application May 4, 1954, Serial No. 427,568

7 Claims. (Cl. 302-66) The present invention relates to the long-distance transportation of coal in water slur-ry form through commercial pipelines. More particularly, the invention relates to a method for reducing metal losses occurring in commercial pipelines employed for transporting coal in slurry form over long distances.

Although coal traditionally has moved from mine to market by railroad, the increasing rail freight rates have resulted in raising the cost of delivered coal to the consumer considerably over the cost of the coal at the mine. Accordingly the coal industry has been seeking less expensive transportation means for coal, in order to reduce the delivered cost of fuel to the consumer. One proposed alternative is pipeline transportation in which coal is prepared into a slurry with water, the slurry is pumped through a commercial long distance pipeline, and dry coal is recovered from the slurry at the pipeline delivery terminal. Such a system has been reviewed in U. S. Bureau of Mines Report investigation 4799, A survey of the hydraulic transportation of coal, by R. W. Daugherty, July 1951.

One of the large expense items in any coal pipeline installation is the actual conduit employed to move the coal slurry. This conduit must have suflicient thickness to withstand the hydraulic pressures employed (operating pressures as high as 1200 pounds per square inch are envisioned for commercial use) over the anticipated useful operating life of the system. Accordingly the conduit also must have suflicient additional wall thickness to accommodate the loss of metal from its inner wall through erosion by the moving slurry and corrosion by the corrosive slurry environment. Erosion metal losses occur through the scouring and scraping action of the abrasive coal particles against the inner wall of the pipe. Corrosion losses occur principally from the dissolved oxygen in the water phase of the coal slurry. If the conduit were constructed of a corrosion resistant material such as stainless steel, the rate of corrosion metal losses could be reduced below the rate of erosion losses so that essentially only the erosion losses would have to be considered. However the cost of using a material such as stainless steel would reduce the economic incentive to substitute coal pipelines for rail freight. In using a relatively inexpensive material such 'as carbon steel for the pipeline, the corrosion metal losses resulting from the transportation of aqueous coal slurry exceed the erosion losses; the overall anticipated metal loss from the inner wall of the pipe would necessitate designing such high corrosion allowances in the pipe wall thickness that the conduit cost once again would reduce the economic advantage of coal slurry transportation systems.

The two-phases (liquid and solids) of the coal slurry combine to magnify the metal loss problems in coal pipelines. Normally in metal corrosion systems, a more or less protective film of the corrosion products is formed over the metal surface where corrosion occurs, tending nited States Patent 2,791,472 Patented May 7, 1957 to minimize the tendencies for further corrosion. However in the presence of abrasive moving solids, any protective film of corrosion products which might form is removed almost at once by the erosive effects of the moving slurry; thus an essentially bare metal surface is exposed to the ravages of corrosion at all times when carbon steel is employed in coal pipeline service with ordinary aqueous coal slurries.

Despite the severity of the combined corrosion and erosion propensities of a coal slurry transportation system which employs carbon steel conduit, we have discovered that the overall metal losses occurring during commercial operation can be reduced significantly, i. e., nearly to the values obtained with stainless steel conduit. We have found that maintaining in the coal slurry a certain minimum concentration of chromate salts having hexavalent chromium and maintaining the pH of the slurry above a critical minimum level will, under specitied hydraulic conditions of velocity, coal size consist and slurry concentration, sharply reduce the metal losses normally associated with transporting a coal slurry through a carbon steel conduit. In accordance with our discovery, it is possible to construct commercial coal slurry transportation pipelines from relatively inexpensive carbon steel without requiring excessive pipe wall thicknesses which would reduce the potential economic advantages which pipeline coal delivery atfords.

For a clear understanding of the present invention, reference should be had to the following description and the accompanying drawings in which:

Figure l is a schematic illustration of a typical coal slurry pipeline transportation system; and

Figures 2 and 3 are graphical illustrations showing the effect of various parameters of a coal pipeline system upon the metal losses in a typical commercial installation.

The typical coal slurry pipeline transportation system shown schematically in Figure 1 presents the environment in which the present invention is carried out. There are three principal stages to the slurry transportation system: the slurry preparation terminal, the pipeline proper with its pumping facilities, and the slurry dewatering terminal. The slurry preparation terminal is preferably located in a coal mining area adjacent to a tipple or coal cleaning plant. Coal selected for pipeline shipment is crushed to a suitable size consist, screened and mixed with Water to form the slurry for transportation. The resulting slurry should contain about 35 to 55 percent coal by weight. Slurry from the preparation stage is introduced into the transportation pipeline and pumped at pressures up to about 1200 pounds per square inch through the pipeline for long distances to the slurry dewatering terminal Where dry, marketable coal is recovered from the slurry for delivery to customers. The impetus for moving the slurry through the pipeline is provided by pumping apparatus installed along the length of the pipeline. When using linear velocities of about 4 to 7 feet per second, as provided hereinafter, a 12-inch diameter commercial pipeline can deliver about 5500 to 8000 tons of dry coal daily. In general pipeline diameters of about 8 to 15 inches are preferred for commercial applications.

The hydraulics associated with the movement of coal slurries through commercial pipelines impose limitations upon the operating conditions of long distance pipelines. Other limitations are imposed by particle attrition, dewatering, slurry preparation and delivered product properties. The discovery of these various critical limitations which are imposed upon any coal slurry transportation pipeline has been described in U. S. patent application Serial Number 388,399, filed October 26, 1953, by John A. :Phinney, James T. Clancey, Thomas J. Regan and 3 Edward J. Wasp, entitled: Transportation of Coal by Pipeline. According to the above-mentioned application, coal slurry pipelines require coal slurries having concentrations of 35 to 55 weight percent-solids which have a size distribution in the range of anominal 6 mesh Tyler screen series x to a nominal 28 mesh Tyler screen series it t) and contain less than 25 weight percent of particles which will be retained upon a 14 mesh Tyler screen. In addition, the transportation linear velocity must be maintained within .the range of 4 to '7 feet per second for long distance commercial pipeline operation, e. g., fifty miles and longer. Within these critical limitations, we have found that carbon steel couduit may be employed for coal pipeline service by following the method set forth herein.

To illustrate the extreme metal losses which can result in carbon steel pipelines in coal transportation service, Figure 2 has been prepared from actual operating information. In Figure 2, the average metal loss from the pipeline inner wall (as inches of average penetration per year) is plotted against the linear velocity of the coal slurry passing through the pipeline for three systems: (A) using carbon steel conduit without inhibitors; (B) using stainles steel conduit without inhibitors; and (C) using carbon steel conduit with a chromate and polyphosphate inhibitor as described herein. Figure 2 indicates that the metal losses increase with velocity for commercially usable coal slurries. tainless steel conduit without inhibitors (B) is shown to exhibit strong resistance to metal losses; nevertheless the cost of stainless steels makes their use in commercial pipelines prohibitive. Carbon steel conduit with uninhibited slurries (A) incurs .such great metal losses that excessively thick pipeline would be required in order to provide satisfactory operating life expectancy. However when carbon steel conduit is used with the inhibitors and the other conditions defined herein (C) the metal losses can be controlled and satisfactory commercial operating life expectancies can be achieved without the expense of stainless steels and without the excessive pipe thicknesses which would be required for carbon steel piping under uninhibited conditions.

The corrosion inhibitor which we have found to be suitable for use in commercial coal slurry pipelines is the chromate ion (CrO4=) containing hexavalent chromium in concentrations above 12 parts by weight (measured as CrOi per million parts by weight of the liquid phase of the slurry. Throughout this specification, references to the concentration of chromate ion will be referred to as p. p. m. and will mean parts by weight of chromate ion (measured as CI'O4=) per million parts of the liquid phase of the slurry.

Chromate salts are well known as corrosion inhibitors of the passivating variety and are widely used in industry to inhibit corrosion of mild steel against the ravages of aqueous systems. In aqueous systems having one phase, the chromate forms a more or less protective film over the metal surface which inhibits corrosion of the steel. Where the corrosive medium is a single phase, there is little tendency for the chromate film to be removed so long as chromate ions are present. In single phase systems, moreover, a thin passivating film forms over the metal suitace and is in turn covered with a second and thicker film which reinforces the corrosion inhibiting properties of the single phase system. This second reinforcing film is usually slow in forming. However, in a two-phase corrosive system (liquid and solids) under agitated conditions, the moving solids scrape the metal surfaces and not only prevent the usual thick reinforcing film from forming, but actually remove the thin passivating protection film to expose the bare metal to corrosive attack by the slurry. The concentration of the chromate must be sufficient to permit a nearly instantaneous refilming of the hated metal areas of the pipe if satisfactory corrosion inhibition is to be achieved. We have 'found that a concentration exceeding 12 parts per million of chromate is effective for maintaining the metal losses of carbon steel pipe at a low level when abrasive coal slurries are passing through the pipeline.

In addition we believe we are the first persons to discover that chromate inhibition has the property of instantaneously reforming a satisfactory passivating film in a moving solids slurry system. Hence in any solids slurry moving through a mild steel pipe, chromate i11- hibits corrosion thereby reducing the normal metal losses provided the chromate concentration of the slurry is sufiicient to provide the required passivating film-forming material and provided the solids are substantially wholly suspended in the slurry and the linear velocity is less than about 15 feet per second.

Preferably the chromate concentration should be maintained at a minimum level of 20 to 25 p. p. m. In order that chromate be used as a metal-loss inhibitor in a coal slurry pipeline, it is essential that the pH of the slurry be maintained in excess of 6.0. At pH values below 6.0 we have found that uneconomically large quantities of chromate must be added to the slurry to afford adequate protection to the pipe metal.

Chromate is particularly effective as a metal loss inhibitor with coal slurries having a concentration of less than about weight percent solids. Although chromates will provide inhibition against metal losses in more concentrated slurries, the quantity of chromate required is excessive. In a series of tests illustrating the eifect of slurry concentration on corrosion inhibition via chromate inhibitors, 28 mesh x 0 coal was prepared into slurries of 50 percent and percent concentrations using chromate as the metal loss inhibitor. The two slurries were pumped at 6 feet per second through a commercial scale pipeline for distances of miles and 60 miles respectively. The results of these tests are summarized in tabular form sumption for identically inhibited coal slurries having different solids concentration increases from 122 p. p. rn. to 417 p. p. m. as the concentration increases from 50 percent to 60 percent, despite the fact that the distance traveled by the 60% slurry was only 60% of that traveled by the 50% slurry. We have found chromate requirements are excessive in those slurries having a solids concentration greater than about 55 weight percent.

The velocity at which a coal slurry moves through a commercial pipeline also aifects the metal loss inhibition of chrom ates.

From a consideration of the pipeline hydraulics, the linear velocity of the slurry should be maintained within the range of about 4 to 7 feet per second. We have found that the chi-ornate inhibition is efiective in reducing metal loss over this range. For an illustration of the effect of varying velocity on the metal losses resulting from a particular coal slurry inhibited with chromatcs, reference should be had to Table II which shows the metal losses (as inches of maximum penetration per year) as a function of the linear slurry velocity (as feet per second). The data for Table II were obtained with a 50% solids concentration slurry having essentially 8 s X sea on a n ng by weight o at ia too large to pass through a '14 mesh Tyler screen).

TABLE II Metal Loss, Maximum Penetration (inches per year) Slurry Velocity (feet per second) With the particular slurry used to obtain the data for Table ll, the metal loss increases with the slurry velocity until the slurry velocity attains a value of about 4 feet per second; thereupon as the velocity increases the metal loss decreases until a value of about 6 feet per second is obtained; thereafter any increase in velocity results in an increase in metal loss. With commercially usable coal slurries moving through commercial pipelines at velocities in the range of 0 to 4 feet per second, a significant quantity of the solids firom the slurry settle out of suspension forming a bed of solids along the bottom portion of the pipe. The moving slurry creates particularly acute corrosion conditions along the side walls of the pipe near the level of the bed of settled solids.

As the flow velocity of the slurry increases in the range of 0 to 4 feet per second, the quantity of solids retained in suspension increases, and the bed of settled solids along the bottom of the pipeline diminishes in size until only a few particles remain out of suspension. These particles probably exhibit a rolling movement along the bottom of the pipeline thereby producing a maximum penetration at that point. While the average penetration of coal slurries moving through commercial pipelines is low in the range of 0 to 4 feet per second, the maxi mum pipe wall penetration is greatest in these low velocities. In the hydraulically satisfactory range of velocities, about 4 to 7 feet per second, total suspension of the solids in the slurry occurs and the pipe wall penetration is generally uniform throughout the periphery of the conduit.

In the hydraulically satisfactory velocity range of 4 to 7 feet per second the chromate film restoring capacity of the inhibited slurry oifsets the film removing tendencies of the moving abrasive solids. At velocities in excess of the hydraulically satisfactory range the film removing tendencies of the moving abrasive solids exceeds the capacity of the inhibited slurry to restore the protective film. Accordingly at higher velocities the protective film is removed at a greater rate than it can be restored. In addition the abrasive forces Work upon the bare metal instead of upon the protective film at higher velocities. However in the range of velocity from about 4 to about 15 feet per second, the total suspension of solids results in a turbulence which serves two valuable functions: (1) the chromate ions are brought into contact with the bared metal surfaces to facilitate film restoration; and

(2) the homogeneity of flow creates a generally uniform distribution of metal wear throughout the periphery of the pipe wall, thereby avoiding areas of penetration.

We have found that this characteristic metal loss versus slurry velocity pattern occurs with nearly all chromate inhibited slurries and that for commercially feasible coal slurries the optimum velocity occurs generally in the range of about 4 to 7 feet per second. The particular optimum velocity will vary with the type of coal, the size consist of the coal and the slurry concentration. Nevertheless, it is only within this narrowly defined range of slurry velocity and particle size that the pipe metal losses can be maintained at a sufiiciently low value to permit the economic construction and operation of long distance commercial coal pipelines with relatively inexpensive carbon steel.

The size consist of the coal in the slurry exerts an influence on the metal loss inhibition which can be expected by the use of chromate inhibitors as will be shown" in connection with Figure 3. The metal losses (as inches of metal penetration per year) of coal transportation pipelines are plotted in Figure 3 against the weight percentage of material in the coal slurry exceeding 14 mesh Tyler screen size. The data tor Figure 3 were obtained at linear velocities of 4, 5, 6, 7 and 10 feet per second. It will be seen that the maximum penetration of inhibited slurries increases as the percentage of solids exceeding a 14 mesh Tyler screen size increases. Similarly, the maximum penetration increases with the linear velocity of the slurry transportation as previously described. For coal slurries containing more than 25 weight percent of solids exceeding 14 mesh Tyler screen size, the maximum pipe wall penetration is excessive. We have found that to maintain the metal losses at a reasonably low level the coal in the slurry should have a nominal top size of less than about 6 mesh by 0 (Tyler screen size) and moreover should contain less than 25 weight percent of material larger than a 14 mesh Ty ler screen.

We have found in addition, however, that the coal size consist should not be extremely fine since the chromate consumption of extremely fine coal slurries is excessive because of [a peculiar ability of very fine coal to remove chromate ions from solution. Where extremely fine coals are employed, the chromate requirements of the pipeline system are excessive, although the metal osses can be maintained at a low value if sufiicient chromate is introduced to compensate for the substantial quantity of chromate removed by the action of the coal itself. In general we have found that coals having nominal size consists not finer than 28 mesh x 0 will not create an excessive demand for chromate addition.

We have further found that the disappearance of chromate salts from coal slurry pipeline systems appears to be a first order reaction in which the disappearance rate is proportional to the chromate concentration. Accordingly it is preferred that the chromate be added to the pipeline system in increments over the length of the line in order that the chromate concentration be maintained only slightly above the desired operating concentration level. The most convenient chromate addition points, of course, are 'the pumping stations which are installed throughout the pipeline to elevate the hydraulic pressure of the moving stream. However additional inhibitorinjection stations may be constructed along the pipeline if desired.

In operating a coal pipeline in accordance with this in-' vention, a large injection of chromate salts is added to the freshly prepared slurry to provide a chromate concentration Well in excess of the desired operating level. The substantial excess is required to compensate for the capacity of the coal itself to remove chromate salts from solution. Thereafter, as the slurry moves through the pipeline, the chromate concentration decreases as the chromate is deposited as a passivating film on the inner pipe wall. Succeeding chromate injections are sufficient to restore the chromate concentration to a level above the desired minimum operating level so that further travel to the next downstream injection point will not reduce the chromate concentration below the desired minimum value. In addition it may be desirable to add a small quantity of caustic to the moving slurry along with the chromate to assure that the pH is maintained above the desired critical minimum value of 6.0.

Hexavalent chromate salts usually are available commercially in the form of dichromates. add caustic to the dichromate salts to convert them to the chromate form. Thus, at the slurry preparation stage, small quantities of caustic will be required to convert the commercially available dichromates to their usable form for this applicationchromates. Additional caustic may be required to maintain the pH of the slurry above the critical minimum value of 6.0. Goals which It is necessary tov have been subjected to weathering or extensive exposure to oxidation in the air will produce acidic slurries and will require even greater quantities of caustic for pH control.

When the coal slurry arrives at the slurry dewatering terminal, steps must be taken to remove the chromate salts from the water phase of the slurry prior to its discharge into public streams. While any convenient means may be employed to reduce the chromate concentration to a level in compliance with stream pollution legislation, we have found that the chromate level decreases to a large extent merely through the contacting of the water phase with the very finely divided coal in conventional slurry thickening equipment. Thus the reagent costs for final clean-up of the discharge water are negligible since the chromate concentration in the ciear cfiluent stream from the thickening equipment is quite low.

The use of chromate as a corrosion inhibitor generally decreases the overall level of corrosion attack on mild steel at the expense of permitting undue pitting to occur in the steel. We have found that the addition of polyphosphates to the slurry along with the chromate will reduce the pitting tendencies of the chromate salts in coal slurry transportation pipelines. A preferred polyphosphate is the sodium hexametaphosphate manufactured and sold commercially under the trade-name Calgon. The polyphosphates are added to the pipeline together with the chromates and preferably are added in an amount equal to the weight of the chromate (measured as NazGrOsAHzO). The use of the polyphosphate coupled with the chromate under the conditions set forth herein is effective in reducing the overall metal losses occurring in coal slurry transportation and in addition is effective in reducing the extent of pitting which occurs in the inner pipe wall.

Thus we have discovered a method of operating slurry pipeline transportation systems for coal which permit the use of ordinary carbon steel having an economically feasible wall thickness. We have found chromate salts to be a satisfactory inhibitor, but also have found that their effectiveness is limited to the described narrow ranges of operating conditions in the pipeline, the coal and the slurry.

And now, according to the provisions of the patent statutes, we have explained the principle, preferred construction, and mode of operation of our invention and have illustrated and described What we now consider to represent its best embodiment. However, we desire to have it understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically illustrated and described.

We claim:

1. A method for transporting coal through commercial pipelines which comprises preparing a 35-55 weight percent solids slurry of water and coal having a nominal top size of 28 mesh to 6 mesh Tyler screen series and having less than 25 weight percent of particles retained on a 14 mesh Tyler screen, pumping said slurry through said pipeline, and inhibiting metal loss from said pipeline by adding to said slurry sufficient chromate salts to maintain a chromate ion concentration of at least 12 parts by weight (measured as CrO4=) per million parts of water in said slurry, and further maintaining the pH of the slurry at a level in excess of 6.0.

2. A method for transporting coal through carbon steel pipelines which comprises preparing a 35-55 weight percent solids slurry of water and coal having a nominal top size of 28 mesh to 6 mesh Tyler screen series and having lessthan 25 weight percent of particles retained on a 14 mesh Tyler screen, pumping said slurry through said pipeline, and inhibiting metal loss from said pipeline by adding to said slurry sufficient chromate salts to maintain a chromate ion concentration of at least 12 parts by weight (measured as CrO4=) per million parts 8 by weight of water in said slurry, and further maintaining the pH of the slurry at a level in excess of 6.0.

3. A method for transporting coal through commercial transportation pipelines which comprises preparing a 35-5 5 weight percent solids slurry of Water and coal having a nominal top size 28 mesh to 6 mesh Tyler screen series and having less than 25 weight percent of particles retained on a 14 mesh Tyler screen, pumping said slurry through said pipeline, and inhibiting metal loss from said pipeline by injecting sufficient chromate salts containing hexavalent chromium into said slurry at selected points in the transportation system to maintain at all times a chromate ion concentration of at least 12 parts by weight (measured as CrOr per million parts by weight of water in said slurry, and further maintaining the pH of the slurry at a level in excess of 6.0.

4. A method for transporting coal through commercial transportation pipelines which comprises preparing a 35-55 weight percent solids slurry of water and coal having a nominal top size of 28 mesh to 6 mesh Tyler screen series and having less than 25 weight percent of particles retained on a 14 mesh Tyler screen, pumping said slurry through said pipeline, and inhibiting metal loss from said pipeline by injecting a sufilcient NazCr04.4H2O

into said slurry at selected points in the transportation system to maintain at all times a chromate ion concentration of at least 12 parts by weight (measured as CrO4=) per million parts by weight of water in said slurry, and further maintaining the pH of the slurry at a level in excess of 6.0

5. A method for-transporting coal through commercial transportation pipelines which comprises preparing a 35-55 weight percent solids slurry of water and coal having a nominal top size of 28 mesh to 6 mesh Tyler screen series and having less than 25 weight percent of particles retained on a 14 mesh Tyler screen, pumping said slurry through said pipeline, and inhibiting metal loss from said pipeline by injecting chromate salts together with polyphosphates into said slurry at selected points in the transportation system to maintain at all times a chromate ion concentration of at least 12 parts by weight (measured as CrO4=) per million parts by Weight of water in said slurry, and further maintaining the pH of the slurry at a level in excess of 6.0.

6. A method for transporting coal through commercial transportation pipelines which comprises preparing a .35- weight percent solids slurry of water and coal having a nominal top size of 28 mesh to 6 mesh Tyler screen series and having less than 25 weight percent of particles retained on a 14 mesh Tyler screen, pumping said slurry through said pipeline, and inhibiting metal loss from said pipeline by adding initially and thereafter injecting at certain selected points in the transportation system sufiicient chromium salts containing hexavalent chromium to said slurry to maintain a chromate ion concentration in excess of 12 parts by weight (measured as CrO4=) per million parts by weight of water in said slurry and suhicient caustic to maintain the pH of said slurry in excess of 6.0.

7. A method for transporting coal through commercial transportation pipelines which comprises preparing a 35-55 weight percent solids slurry of water and coal having a nominal top size of 28 mesh to- 6 mesh Tyler screen series and having less than 25 weight percent of particles retained on a 14 mesh Tyler screen, pumping said slurry through said pipeline, and inhibiting metal loss from said pipeline by adding initially and thereafter injecting at certain selected points in the transportation system sufficient chromate salts containing hexavalent chromium to said slurry to maintain a concentration in excess of 12 parts by weight (measured as CrO4=) per million parts by weight of water in said slurry, sufiicient caustic to maintain the pH of said slurry in excess of 6.0, and suflicient polyphosphates to maintain a pipewall pitting inhibiting concentration.

References Cited in the file of this patent UNITED STATES PATENTS Greenstreet Feb. 3, 1920 Krekeler May 26, 1936 McConnell Oct. 3, 1944 Cross Sept. 16, 1952 Cross Sept. 16, 1952 Odell Aug. 10, 1954 

1. A METHOD FOR TRANSPORTING COAL THROUGH COMMERCIAL PIPELINES WHICH COMPRISES PREPARING A 35-55 WEIGH PERCENT SOLIDS SLURRY OF WATER AND COAL HAVING A NOMINAL TOP SIZE OF 28 MESH TO 6 MESH TYLER SCREEN SERIES AND HAVING LESS THAN 25 WEIGHT PERCENT OF PARTICLES RETAINED ON A 14 MESH TYLER SCREEN, PUMPING SAID SLURRY THROUGH SAID PIPELINE, AND INHIBITING METAL LOSS FROM SAID PIPELINE BY ADDING TO SAID SLURRY SUFFICIENT CHROMATE SALTS TO MAINTAIN A CHROMATE ION CONCENTRATION OF AT LEAST 12 PARTS BY WEIGHT (MEASURED AS CRO4=) PER MILLION PARTS OF WATER IN SAID SLURRY, AND FURTHER MAINTAINING THE PH OF THE SLURRY AT A LEVEL IN EXCESS OF 6.0 